A statically-linked copy of Pacman (the Arch package manager). Ideal for rescuing systems where libc is messed up.
A FOSS utility for investigating Tor hidden services. Searches for configuration errors and information leaks. Written in golang. In the AUR. CLI tool but the Onionscan Correlation Lab is a webapp that lets you analyze the data visually.
The more-or-less official file format for offline reading of Wikipedia. The full byte-by-byte description of ZIM archives can be found here. There is also a zimlib-git AUR package for Arch Linux.